I also have had trouble installing AVAST 2014. I did have Malwarebytes Pro (Trial) installed and in task bar. The only way I could install was in SAFE Mode.

Why can’t Avast be installed in safe mode?

I will try to disable Malwarebytes then install Avast 2014 in normal mode.

Avast will have to be uninstalled in safe mode using Avast uninstall utility. It does not uninstall normally.